[FTS] Add indexing filters on keys

Enable a regular expression on the key / id to define the type of a document, or otherwise select / exclude documents to index by specifying a regular expression. Related to MB-17846

Customer stores older versions of "product::” documents using a version suffix on the key (e.g. “products::A:1“) and a per document ":id” document to keep track of the next version numbers.

Customer doesn't want to index the older versions of the products.

This can be done using the “advanced filter” feature in the XDCR transport between CB and ElasticSearch to index only the most recent versions of “product::” documents.

This is accomplished by using a regular expression that encodes the form of keys that are valid for indexing.

Request: make it possible to specify document type by providing a reg-ex key pattern. This would mean “any document with a key matching this regex has type X.”